The use of electrical arcing in surgical procedures has been known for some time. In one external application, an electrical scalpel, sometimes referred to as a Bovee knife, is employed to simultaneously make an incision and cauterize the incised tissue. In the use of that apparatus, a surgeon can visually observe the arcing between the knife and tissues and the cutting of the tissue. The surgeon can make adjustments in the position of the scalpel or in the electrical signal for controllably making and cauterizing an incision. For example, if the arc is observed to be too strong, either the electrical energy supplied to the scalpel can be reduced or the scalpel can be moved to a position farther from the tissue.
Electrical ablation of occlusions within lumens, such as arteries and vessels, has also been known for some time. In that technique, a wire, catheter, or other implement, generically referred to here as a wire, having an electrode at its end is inserted into a vessel or artery and moved to a position adjacent an occlusion. Once an occlusion is encountered, electrical energy, usually in the form of pulses, is supplied to the electrode so that arcing occurs. The plaque forming the occlusion is vaporized or reduced to very small particles if the arcing ablation proceeds as intended. In some apparatus, for example, the type described in U.S. Pat. No. 4,682,596 to Bales et al, a bipolar catheter is employed. In a bipolar catheter, two wires are inserted in the lumen and two electrically isolated electrodes are present at the end of the catheter. Arcing occurs between the two electrodes. In other known apparatus, an example of which is described in PCT Application W090/07303 to Janssen, a monopolar, rather than bipolar, catheter may be employed. A monopolar catheter is used in conjunction with a dispersive electrode or ground pad that is placed on a portion of an animal's body, such as a human's belly or thigh, which provides an electrical return path. Arcing then occurs between a single electrode at the end of the catheter and the grounded body. Janssen also discloses a bipolar arcing catheter apparatus.
In addition to occlusion removal, percutaneous electrosurgery may be carried out in other liquid-containing body cavities or lumens where visual observation is difficult or impossible. For example, arthroscopic procedures may be used for releasing or shaping ligaments. In laparoscopic techniques, nerves may be severed, tissues may be incised, and parts or all of organs removed through a relatively small incision that is far less invasive than conventional surgery. Urological surgery, such as transurethral resection of the prostate and ablation of cancerous tissues, also may be carried out using electrosurgical techniques. Vascular ablation has already been extended to plaque ablation within the heart in the presence of blood. In all of these procedures, the cavity or lumen in which the electrosurgery takes place is partially or completely filled with a fluid, such as blood or a saline solution, that affects and usually interferes with the electrosurgery.
There is a significant difference between external electrosurgery, such as the use of the Bovee knife, and other internal vascular electrosurgery, such as occlusion ablation. In internal electrosurgery, it is impossible to observe the arcing causing plaque removal or cutting of tissues. In fact, it is even difficult to determine the position of the electrode where arcing is taking place. Janssen suggests the use of ultrasound to determine the location of the electrode. Other techniques include adding a contrast medium for fluoroscopic observation of the position of the electrode. While these and other techniques may permit determination of the location of an electrode, they do not permit observation or control of arcing to ensure that an arc occurs and has particular qualities.
Producing an arc, particularly a monopolar arc, within a liquid, such as a saline solution or blood, presents difficult problems. For example, the efficiency of the arcing and tissue removal decreases significantly as compared to external electrosurgery. The typical response to this efficiency problem with known electrosurgical apparatus is an increase in the electrical power applied to the electrode. However, increased power may cause damage to patient tissue remote from the surgical site, increasing the risk that the surgeon will receive an electrical shock, raising the probability of undesired tissue charring or excessive incision, and may cause loss of sensitivity in the surgeon's control.
It is well known that the electrical impedance of an electrosurgical electrode and connecting wire varies depending upon the position, i.e., depth of insertion, relative to a body, the quantity of adjacent liquid, if any, and other variable factors. The prior art has not taken into account the varying load impedance as a wire and electrode are advanced in a body cavity or lumen or the effect of the impedance change on the energy of an arc and the resulting surgical process. In an electrosurgical scalpel application, i.e., in a dry environment, it has been recognized that, as moisture is driven from tissue by bipolar arcing, impedance increases and can result in problems such as adhering of instruments to tissue (see, for example, U.S. Pat. No. 4,658,819 to Harris et al).